Gouges and Gashes: Horse Wounds

They are the signs no horse owner wants to see: trails of blood across stall or pasture, clumps of hair caught on a wire or nail, dangling skin flaps, unexpected lamenesses, or swollen body parts. Yet horses and wounds seem to find each other with distressing regularity. Wound outcome correlates with management approach, even from the beginning, so understanding wound types and the wound healing process and pursuing proper treatment are central to optimal healing.

Wound Types
Though wounds can, and do, occur on any part of the equine integument (covering)--skin, cornea, or hoof—this article will cover only wounds to the skin and underlying tissues.

Abrasions are partial thickness wounds (think road rash or a scraped knee). They can be large and become quite contaminated, but they do not fully penetrate all the layers of skin.

Puncture wounds result from penetration with foreign objects--commonly nails, tree branches, or pieces of wire. They are narrow in diameter relative to their depth. Usually, they bleed very little and for that reason smaller punctures might not be noticeable immediately.

Lacerations are full-thickness wounds that transect the skin completely and often extend into underlying tissues.

Sharp or pointed foreign objects or ragged edges create most of these wounds, but blunt trauma (kicking, falling on, or running into a solid object), entrapment (catching a body part between objects such as rope, wire, or panels), and penetration/impalement (think chest wounds via T-posts and large branches) also can cause open wounds. According to Ted Stashak, DVM, MS, Dipl. ACVS, professor emeritus of surgery at Colorado State University, the cause of injury is a major player in wound management and probable outcome. Some of the reasons for this become clear as we look at the healing process.

Phases of Wound Healing
Wound healing occurs in three phases: the acute inflammatory phase, the proliferative phase, and the remodeling phase.

Although we tend to think of "inflammation" as a bad word, involving such undesirable signs as redness, pain, and swelling, it is the body’s first line of defense against infection. Think of the inflammatory phase as the cellular version of spring cleaning prior to rebuilding the damaged tissue. Several things happen during the inflammatory phase: Cells lining the damaged blood vessels' interior release chemical mediators that begin the inflammatory cascade. These chemicals signal the blood vessels to constrict initially--slowing bleeding--then the small vessels dilate, allowing white blood cells, fluids, and proteins to pass into the tissue space.

The mediators also pull blood platelets and fibrin, a protein, to the area, allowing a clot to form that helps staunch blood flow and forms a scaffold for the initial inflammatory cells and proteins to stay at the site of injury and begin the healing process. Later, the clot turns into a scab and helps seal the wound and prevent infection.

The white blood cell invasion comes in two waves: The first is comprised primarily of neutrophils whose enzymes destroy bacteria and debris. After the neutrophils come the macrophages, which not only clean up dead neutrophils, bacteria, and other debris but also help regenerate tissue by calling in the cells that repair wounds.

The proliferative phase begins as inflammation subsides, usually after four or five days. In a full-thickness wound this phase is characterized by the development of granulation tissue, which is comprised of macrophages, fibroblasts (connective tissue cells), and new blood vessels and looks like a pink sea sponge with a cobblestone texture. The granulation tissue replaces the scab as a barrier to infection, filling the defect and providing a surface new cells will migrate across in a subsequent process called epithelialization. During this time, as the wound matures, tissue-strengthening collagen forms.

In a partial thickness wound such as an abrasion, this process is much faster, since there isn’t a defect to be filled with granulation tissue first and epithelial cells from surviving hair follicles and glands contribute to healing.

Proper Wound Prep

1. Clip hair around the wound margins. Use moistened gauze sponges to keep hair from falling into the wound.

2. Irrigate (rinse) the wound with saline or another isotonic solution rather than tap water if contemplating suture. Irrigating at the proper pressure (8-15 psi) and at an oblique angle (not 90 degrees, directly at the wound) can loosen debris and remove bacteria from the surface.

3. Apply a topical antibiotic/antiseptic.

4. Depending on the location, bandage the wound to prevent further contamination.

5. Call your veterinarian.

--Christy Corp-Minamiji, DVM

The proliferative phase doesn’t always have a tidy, timely resolution: Stashak notes that prolonged inflammation of wounds can lead to disproportionate, unregulated fibroblast proliferation. The result is exuberant granulation tissue, otherwise known as proud flesh (more on this later).

The remodeling phase (ideally) follows the proliferative phase, beginning about three weeks after injury and lasting for at least a year. The most outwardly visible sign of remodeling is wound contraction, which minimizes the eventual scar size. Contrary to popular belief, scar tissue is not stronger than normal tissue; scar tissue cells are less organized, structurally weaker, and lack many normal features such as hair follicles and glandular tissue.

When Good Wounds Turn Bad
When anything disrupts the described phases, the healing process can go awry. "Infection is a major player in delaying wound healing and in the formation of exuberant granulation tissue," says Stashak. "It is a major player in dehiscence (the reopening of a wound that has been sutured). Bacteria (initially) sit on the surface of a wound. After about three hours, the bacteria begin to invade the local tissue and then injure the host."

In creating an environment suitable to their own survival, bacteria "release enzymes that damage tissue, alter the blood supply, cause thrombosis of the microvasculature (blood clot formation in blood vessels), and compete with the host cells for metabolites," Stashak adds.

Limb wound contamination and infection, primarily below the knee or hock, can lead to the prolonged inflammation, unregulated fibroplasia (formation of fibrous tissue), and proud flesh formation as described earlier. In this case the signal that would normally turn off granulation tissue production is interrupted, and cells continues producing the tissue.

Chronic irritation is another factor contributing to proud flesh development, says Stashak. This irritation can come from excessive movement (think of how a cut over a knuckle feels) or exposure to harmful chemicals. While a variety of products are marketed for treating wounds and "preventing" proud flesh, not all are created equal, and some don't belong within a mile of a wound, he explains.

For instance, soaps, strong disinfectants, and caustic chemicals can exacerbate tissue irritation and cellular damage. "As implied by the word, caustic products are detrimental to cells," explains Christine Theoret, DMV, MSc, PhD, Dipl. ACVS, of the Faculty of Veterinary Medicine at the Université de Montréal. "They may variably kill the cell or alter some form of its behavior: migration, proliferation, protein synthesis, etc. Moreover, some caustic products may be cancerogenic."

Then there's the impact of wound cause and location. While a sharp edge might damage tissue and blood supply relatively minimally, entrapment injuries cause greater damage to the blood supply, states Stashak.

Likewise, limb injuries tend to heal more slowly than body wounds. "Data from my lab suggest that a relative state of deficient blood flow exists in wounds at limb level, which may alter tissue perfusion, promote tissue hypoxia (lack of oxygen), and jeopardize repair at the site," says Theoret.

Sutures, Bandages, and More
While we cannot control where or how a horse injures himself, we can respond and treat accordingly for the best outcome. Veterinarians often use sutures in wound management. However, the question of when or whether to suture a wound rears its head more often than not. In a perfect world, suturing closely aligns the tissue, minimizes the surface area that needs to heal, and helps provide a barrier against contamination and subsequent infection.

Says Stashak, some factors that might influence a practitioner's decision on when or whether to suture a wound include:

Cause of injury Greater tissue trauma means greater disruption to the blood supply and greater chance of suture failure. Entrapment injuries traumatize tissue the most and often cannot be sutured because of tissue loss.

Contamination Here the owner plays a major role. "Getting contamination out of a wound and protecting it (increases the) window of time for successful treatment," says Stashak. "If the owner waits until the veterinarian can get there, you have a decreased chance of success."

Type of injury Of degloving (involving the peeling off of skin) injuries, Stashak says, "There's no way you want to suture those initially. They are 100 times more susceptible to infection." With blunt trauma that produces tissue edema (fluid swelling), says Stashak, "You need to wait until the inflammation decreases."

But, you might ask yourself, don’t wounds have to be sutured within a few hours of injury if at all? Not necessarily, says Stashak. Sometimes he opts to bandage a wound and administer systemic antibiotics until the inflammation and local infection subside. Then he debrides (removes) the damaged tissue--essentially creating a fresh wound--and sutures it.

Wound Fact

Studies have shown that ponies' wounds heal more quickly and with fewer complications than comparable wounds in horses. "Their influx of white blood cells is far greater than for horses," explains Stashak. "Therefore, they are less likely to get low-grade infection."

Bandages, splints, and casts can be used to reduce movement of the injured area. But because inappropriate bandaging can cause further tissue damage, owners should seek veterinary assistance.

What about wounds that cannot be bandaged but are inappropriate for suture (e.g., an old, potentially infected wound or one located in a heavily muscled high-motion area)? Preventing contamination of and infection in these wounds becomes paramount. Avoid scrubbing with harsh soaps or applying aforementioned caustic agents. While it might be tempting to just grab the nearest jar of ointment from the feed store, discuss options with your veterinarian first. Researchers have studied a variety of substances that veterinarians might use in wound healing including acemannan (an aloe vera derivative), sugar, manuka honey, maltodextrin, and even maggots.

As researchers in the field of regenerative medicine continue investigating physiologic mediators such as platelet-rich plasma (PRP) for wound management, treatment possibilities might expand even further. "We recommend (PRP) be used in wounds with important tissue deficits (avulsion--all layers of skin torn away--or degloving injuries) since it enhances fibroplasia," says Theoret, who is working to develop a wound healing method using stem cells. "It should be used during the early phases of healing and stopped as soon as granulation tissue has filled the defect."

Wrapping It Up
Wounds might be alarming and expensive for the owner and painful and dangerous for the horse, so proper treatment approach is key. For the best outcome, follow these wound managementing steps:

About the Author

Christy Corp-Minamiji, DVM, practices large animal medicine in Northern California, with particular interests in equine wound management and geriatric equine care. She and her husband have three children, and she writes fiction and creative nonfiction in her spare time.

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